With the current desire for fuel efficient and low emission vehicles, many novel solutions for internal combustion engine architecture and operating strategies have been developed. One such idea is the Belt Alternator Starter (BAS) system. This system provides increases in fuel economy by shutting off the engine when at an idle operating mode, and enabling early fuel cut-off during decelerations. The BAS system can also accommodate regenerative braking. The BAS system combines engine controls with a combined alternator/starter motor mounted with respect to the engine in a typical accessory drive position. As such, this BAS strategy has minimal impact on engine and transmission architectures when compared to other hybrid strategies.
The typical automotive accessory drive system consists of a drive pulley connected to an output shaft of the engine, typically the crankshaft. Wrapped around this pulley is a flexible drive belt, which in turn is wrapped around a plurality of driven pulleys. This flexible drive belt transmits drive forces between the drive pulley and the driven pulleys. The driven pulleys may be fixably attached to accessories known in the art such as a power steering pump, air conditioning compressor, alternator, and secondary air pump. However, some of these driven pulleys may be idler pulleys which may be used to ensure proper belt wrap of a given driven pulley or they may be used to ensure proper belt routing.
The BAS system employs a combined starter/alternator motor mounted with respect to the other components of the accessory drive system. This system combines the alternator and starter motor into one device that can be mounted in effectively the same way and in effectively the same packaging space as a traditional alternator. The BAS system must be able to effect a restart of the engine quickly and quietly. The engine is cranked by the combined alternator/starter motor unit whose driven pulley is linked with the engine's output shaft mounted drive pulley by a flexible drive belt. This flexible drive belt has a tension or taut side and slack side while the engine is running. A flexible drive belt tensioner is typically employed to maintain tension on the slack side of the flexible drive belt. Without this tension, the flexible drive belt may slip, which in turn may cause “belt squeal” or, in extreme cases, belt damage and loss of accessory function. A typical flexible drive belt tensioner consists of an idler pulley which is in communication with the flexible drive belt, a spring to provide the necessary force to the idler pulley, and a viscous damper operable to dampen any resonances that may be induced in the tensioner by the flexible drive belt.
When a request is made to restart the engine, the driven pulley mounted to the combined alternator/starter motor will impart the rotational force necessary to crank the engine to the output shaft mounted via the flexible drive belt. Consequently, the normally slack side of the flexible drive belt becomes the tension side during engine restart. To maintain the required frictional force between the flexible drive belt and the drive and driven pulleys during restart, a flexible drive belt tensioner with a very high spring load is often employed. This spring load is much higher than would be necessary to maintain slack side tension for a non-BAS system or the BAS system while the engine is in running mode.